1. Field of the Invention
The invention relates to devices for directing the flow of a cooling fluid in a cooling channel of an internally cooled blade of a turbomachine, in particular a gas turbine.
2. Discussion of Background
The efficiency of turbomachines, in particular gas turbines, can be improved via an increase in the pressure and in the temperature of the fluid as parameters determining the cyclic process.
The fluid temperatures which are normal nowadays during the operation of turbomachines, in particular in the turbine inlet region, are already markedly above the permissible material temperatures of the components. In this case, in particular the blading of the turbine is directly exposed to the hot fluid flow. As a rule, the heat dissipation of the turbine blades, which is brought about by the heat conduction of the material, is not sufficient in order to avoid an excess temperature of the blades. Material temperatures which are too high first of all lead to a drop in the strength values of the material. In the process, crack formation often occurs in the components. In addition, in the event of the melting temperature of the material being exceeded, local or even complete destruction of the component occurs. In order to avoid these fatal consequences, it is therefore necessary to additionally cool, in particular, the turbine blades of a turbomachine.
The predominant conventional cooling method used nowadays for the cooling of blades by means of a cooling fluid, usually cooling air, is so-called convection cooling. In this case, the cooling fluid is directed through the blades, which respectively are of hollow design or provided with cooling channels. As a result of the lower temperature of the cooling fluid compared with the temperature of the blade material, a heat transfer occurs between the blade material and the cooling fluid as a result of forced convection in the cooling channels. With efficient cooling, the resulting material temperature is therefore below the maximum permissible temperature of the blade material.
At the end of the cooling channel, the cooling fluid mostly flows out into the main flow via one or more openings in the blade wall. Often, however, the cooling fluid is also directed at the end of the cooling channel into a further, internal chamber and passes from there into a further cooling channel or also into the main flow.
So-called film cooling is another method of cooling blades. In this case, a cooling fluid, usually also cooling air, which is supplied in cooling channels, is blown out through openings in the blade onto the blade surface. In the process, the cooling fluid forms a separating layer, similar to a fluid film, between the blade wall and the hot flow fluid. Thus, no direct heat transfer occurs between the hot fluid of the main flow and the blade.
Both methods have the disadvantage that the blades is not cooled uniformly everywhere. In the case of convection cooling, the heat transfer is directly dependent upon the flow conditions in the cooling channels. Higher flow velocities of the cooling fluid increase the heat transfer. In this case, regions in the blade tip in particular are often at a disadvantage, since here, in particular along the cover wall closing off the blade, there are regions having only very low flow velocities of the cooling fluid or also wake zones. It has been possible to compensate for these disadvantages hitherto only by means of very complicated shapes of the cooling channels in the blade. The production of such blades is extremely complicated and thus expensive. In addition, on account of the production of the blades by casting, one or more openings also generally remain in the blade walls, these openings having been necessary during the casting in order to fix the casting core.